The fact that the walking surface is at a low level facilitates passenger entry and exit, thus reducing on the one hand passenger fatigue and on the other hand the halt time for their entry into the carriage, with consequent reduction in the commercial speed.
Carriages comprising two bogies with a depressed floor in the zone between the two bogies have been constructed and are in service, but the depressed floor part represents only a limited proportion of the carriage floor lying between the parts which are raised above the two end bogies.
Recently, in order to construct articulated tramcars with a depressed floor in the zone between the two motorized end bogies, use has been made of articulated systems comprising a conventional bogie with very small wheels which is disposed in proximity to the articulated connection, or comprising a traditional bogie with idle wheels and a very low central thrust bearing.
The currently known articulation systems with a depressed floor have numerous drawbacks, including the following.
The extent by which the carriage walking surface is lowered is not such as to reach a level above the rail which allows entry to the carriage without the aid of at least one raisable or retractable step structure. The result is that the ease and speed of entry of the passengers is only slightly increased, and with it the commercial speed, due to the presence of two step rises for entry purposes. PA1 Care must be taken that the step structure does not emerge where raised pavements are provided at stopping points. PA1 Precautions must be taken against the passenger becoming caught in the step structure so that it cannot be retracted or raised, with the danger of it being able to drag the passenger along when the carriage moves. PA1 The use of small-diameter wheels to obtain this lowering reduces safety both with regard to the wheel loading capacity and with regard to derailment due to the reduced guiding capacity of the wheel flange in reduced-pitch bogies. PA1 The use of braking by means of discs when small-diameter wheels are used is difficult because of the small diameters of the discs themselves, which have to satisfy the prescribed clearances from the rail level. PA1 If the braking is by means of shoes acting on the wheel rolling face, it is practically impossible to use elasticized wheels with rubber inserts because of wheel heating on braking. PA1 The difficulty experienced by handicapped persons in entering the carriage.